Electronic Spectra of PS I Mutants: The Peripheral Subunits Do Not Bind Red Chlorophylls in Synechocystis sp. PCC 6803

• Published in: Biophysical journal Vol. 76; no. 5; pp. 2711 - 2715
• Main Authors: Soukoulis, Victor, Savikhin, Sergei, Xu, Wu, Chitnis, Parag R., Struve, Walter S.
• Format: Journal Article
• Language: English
• Published: United States Elsevier Inc 01-05-1999; Biophysical Society
• Subjects: Cellular biology; chlorophyll; Chlorophyll - chemistry; Chlorophyll - genetics; Chlorophyll - metabolism; Cyanobacteria - chemistry; Cyanobacteria - genetics; Mutation; Photochemistry; Photosynthetic Reaction Center Complex Proteins - chemistry; Photosynthetic Reaction Center Complex Proteins - genetics; Photosynthetic Reaction Center Complex Proteins - metabolism; photosystem I; Protein Binding; Protein Conformation; spectral analysis; Spectrometry, Fluorescence; Spectrophotometry; Spectrum analysis
• ISSN: 0006-3495; 1542-0086
• DOI: 10.1016/S0006-3495(99)77423-0

Steady-state fluorescence and absorption spectra have been obtained in the Q
y spectral region (690–780
nm and 600–750
nm, respectively) for several subunit-deficient photosystem I mutants from the cyanobacterium
Synechocystis sp. PCC 6803. The 77
K fluorescence spectra of the wild-type and subunit-deficient mutant photosystem I particles are all very similar, peaking at ∼720
nm with essentially the same excitation spectrum. Because emission from far-red chlorophylls absorbing near 708
nm dominates low-temperature fluorescence in
Synechocystis sp., these pigments are not coordinated to any the subunits PsaF, Psa I, PsaJ, PsaK, PsaL, or psaM. The room temperature (wild-type–mutant) absorption difference spectra for trimeric mutants lacking the PsaF/J, PsaK, and PsaM subunits suggest that these mutants are deficient in core antenna chlorophylls (Chls) absorbing near 685, 670, 675, and 700
nm, respectively. The absorption difference spectrum for the PsaF/J/I/L-deficient photosystem I complexes at 5
K reveals considerably more structure than the room-temperature spectrum. The integrated absorbance difference spectra (when normalized to the total PS I Q
y spectral area) are comparable to the fractions of Chls bound by the respective (groups of) subunits, according to the 4-Å density map of PS I from
Synechococcus elongatus. The spectrum of the monomeric PsaL-deficient mutant suggests that this subunit may bind pigments absorbing near 700
nm.